Field of the Invention
This invention relates to centrifuges and, more particularly, to novel systems and methods for centrifugal separators.
Background Art
A centrifuge has the characteristic of spinning a material, and relying on the angular velocity to increase an effective weight of the contained material. That is, the weight of a material in a gravitational field is related to mass times the acceleration of gravity, one “g.” In contrast, the angular acceleration of a material or a body is in the direction of the center rotation.
It is a principal of physics that an angular velocity is typically measured in radians, and every full revolution contains two Pi radians. In general, any point undergoes a radial acceleration equal to the radius at which acceleration is measured multiplied by the angular velocity squared. Thus a wheel, a centrifuge, or any other material or object rotating about a rotational axis has an angular acceleration proportional to the angular velocity squared multiplied by the radius at which acceleration is measured.
Centrifuges have been known and used for over one hundred years in various industrial applications. For example, a cream separator provided continuous, two-dimensional separation by exposing raw milk to a centrifuge having discs. Discs were actually shaped as frustums of cones. That is, each was conical in shape, but truncated nearest the point or vertex. Centrifugation of milk caused a migration of heavier species (the higher water content of skim milk) downward under the influence of gravity and outward under the influence of the centrifuge). At the same time, cream was separated as the lighter, fat-based, cream species inward and upward.
Centrifuges are used in batch mode as well. For example, hospital laboratories maintain centrifuges for separating out blood constituents in preparation for tests, and the like. Many other industrial centrifuges are similarly used in batch mode.
Typically, centrifugal separators operating on a continual basis are cylindrical in geometry, must vent gases exuded by the liquids therein, and must be open to vent gases into the atmosphere or some collection system.
Industrial centrifuges have included cyclones, wherein the centrifugal forces are not induced by moving parts. For example, a conventional saw mill introduces sawdust in a flow of air progressing tangentially along and inside an outer wall of a cyclone. Heavier particles, of the sawdust, tend to react to the requirement for angular acceleration towards the center, by striking the outer wall, slowing down, and thus dropping to the bottom of the cyclone. Meanwhile, the lighter air which can turn as required by the flow constraint of the wall, typically accumulates towards the center, and rises out the top of the cyclone.
Centrifugal separators, in which the entire cylinder of the centrifuge is rotating, may similarly by used for separating out species, whether the species are gases in liquids, liquids in liquids, liquids in gases, gases in gases, liquids in solids, or solids in liquids. Thus, in general, whether single phase or multi-phase, centrifugal separators may migrate a heavier species toward an outer wall of a spinning chamber, while migrating a lighter, lower-density species toward the center. Typically, the ability to control the speed or angular rotational rate of a centrifuge allows virtually any acceleration (number of g's) that can be engineered into the rotating structure and its drive mechanisms.
Continuous centrifugal separators, typically require routine, stationary, highly invasive cleaning procedures. Just as sediment sinks to the bottom of a pond, heavier species, particularly solid matter, such as grit, waste, trace minerals, precipitates, and the like that are heavier than the principal fluid in which they dwell, will be driven toward an outer wall. They may or may not be entrained to exit with the heavier species.
Thus, the accumulation of a certain amount of undesirable debris against an outer wall is common. The common solution is to periodically cease operations, dismantle the centrifuge, and clean or replace components that have accumulated the waste products. Moreover, inasmuch as precipitates, grit, and other solids tend to be abrasive, parts may require replacement on a regular basis, due to abrasive deterioration.
Industrial centrifuges, and particularly continuous centrifugal separators, are the bane of maintenance engineering and staff in an industrial plant. They tend to be maintenance intensive. The maintenance requires skill, down time, service time, re-work time, as well as tools, cleaning equipment, parts inventory, and so forth.
It would be an advance in the art to be able to provide a self-cleaning, centrifugal separator. It would be a further advance in the art to provide effective dynamic cleaning, not having to shut down operation of a continuous centrifugal separator. It would also be an advance in the art to provide mechanisms in a continuous centrifugal separator to minimize accumulation of waste products, to test in some way the operational characteristics of a continuous centrifugal separator, to test certain fluid properties of materials entering and exiting such a separator, to assess condition, schedule maintenance, and schedule routine dynamic cleaning, static cleaning, or the like.
In the prior art, it is common understanding that continuous centrifugal separators are unable to process variable flow rates, or tolerate substantial variation in the constituents of an incoming (influent) stream. They are unable to move off their operational design point to change feed ratios, back pressures, or the like on the flow streams exiting. Rather, systems are engineered, set up, and operated as open-loop systems in which conditions of the equipment, materials processed, or both are not fed back to alter the operational characteristics of the system or process. Thus, it would be an advance in the art to improve any or all of these limitations on continuous centrifugal separators. It would be a further advance in the art to provide a precise and effective control mechanism for controlling operation of a continuous centrifugal separator
Two operational characteristics of a centrifuge are the rotational (angular) velocity and flow rate. Angular Velocity may be written in terms of revolutions per minute, revolutions per second, radians per second, or the like. Typically, a centrifuge is built to operate at a particular design frequency, the rotational speed as revolutions per minute. Many centrifuges are not operated at any significantly degraded frequency.
Moreover, the flow rate through a centrifuge is also a matter of engineering design. Typically, the volumetric flow rate or mass flow rate through a centrifuge is a design characteristic engineered to match the rotational speed or angular velocity of the centrifuge. The turn down ratio of a volumetric flow rate or mass flow rate is the ratio calculated as the value of a reduced set point, divided by the design set point for volumetric flow rate. That ratio permissible is conventionally extremely limited.
For example, turn down ratios for volumetric flow rate are typically not less than about seventy percent. That is, volumetric flow rate may not be turned down more than about thirty percent, leaving a net throughput of about seventy percent of the original design rated value.
It would be an advance in the art to design a centrifuge system and a design process by which a continuous centrifugal separator could be operated off the specified nominal design parameters, at a wide range of turn down ratios. Thus, it would be an advance in the art to provide a system tolerating longer residence times of fluids passing through the separator, thus providing a better control of the “cut” in response to changes in influent feed stock or the like.